Hybrid electric vehicles achieve high fuel efficiency and low emissions by combining small, highly efficient internal combustion gasoline engines with electric motors. Although the mechanical means by which the electric motor and gasoline engine are coupled varies between vehicle manufacturers, almost all hybrid electric vehicles utilize both the gasoline engine and the electric motor to power the driving wheels. The engine control system on the vehicle varies the amount of power from the electric motor and the gasoline engine depending on necessary power output and driving conditions, selecting the most efficient method of powering the car for the situation at hand.
In general, fuel efficiency in hybrid electric vehicles is enhanced by minimizing use of the gasoline engine at inefficient periods such as when the vehicle is temporarily stopped. Such vehicles increase fuel efficiency by shutting off the gasoline engine at extended stops, such as at stop signs or stop lights (this is known as an ‘extended engine off’ situation). When the gasoline engine is off, auxiliary systems such as the radio, gauges, power windows, and the like are kept operative by a low voltage (usually 12 volt) electrical system. When the stop light changes or when it is otherwise safe to proceed, the accelerator pedal is depressed, the gasoline engine starts up immediately, and the vehicle can drive off. Such extended engine off operation is beneficial in reducing fuel use, but makes operation of a conventional climate control system difficult. The passenger cabin heating and air conditioning systems do not work without some kind of power input. The compressor that powers the air conditioning system runs off of the crankshaft of the gasoline engine, and therefore is inoperative when the gasoline engine is shut off at stoplights or stop signs. Without the compressor running, pressure differentials within the air conditioning system, that are necessary for the air conditioner to function, quickly decrease, eliminating the cooling ability of the air conditioner. Without the cooling ability of the air conditioning system, the air circulating through the passenger cabin increases in temperature, may become uncomfortably warm, and, after a few seconds, begins to have a musty smell. The passenger cabin heating system also does not work without the gasoline engine running. The heater core is heated by engine coolant that circulates through and flows from the gasoline engine. When the gasoline engine is turned off, the coolant no longer circulates, and the heater core is no longer able to warm the air that flows to and warms the passenger cabin.
Conventional hybrid electric vehicles deal with this extended engine off climate control problem in a number ways. One method is to simply take no action. When the vehicle arrives at a stop sign or stoplight, the gasoline engine turns off, and the vehicle provides the occupants of the passenger cabin with no additional heating or cooling until the accelerator pedal is depressed and the gasoline engine starts again. This approach is economical, but may lead to uncomfortable conditions for the vehicle passengers. Another approach to the extended engine off climate control problem is keep the gasoline engine running at stoplights or stop signs. Keeping the engine running allows the climate control system to continue providing the passenger cabin with heating or cooling, but contributes nothing to fuel efficiency as the gasoline engine is still operating and consuming fuel. A third approach to dealing with this problem is employed by some “mild” gasoline-electric hybrid engines having a combined electric starter-alternator motor that supports the hybrid functionality. This unit is typically belted to the crankshaft pulley of the gasoline engine to perform the automatic engine shutoff, automatic restart, and charging functions. If the crankshaft pulley is actually clutched to the crankshaft, the associated belt driven components (e.g., air conditioning compressor and engine coolant pump) can be driven by this electric motor when the gasoline engine is in a temporary shutoff state. This allows the passenger compartment to continue receiving cooling or heating air flow. The maximum fuel efficiency of the hybrid vehicle is reduced, however, because the battery energy that powers the electric motor must be replenished, at some time, by the gasoline engine.
Currently, the only ways to maintaining passenger comfort during an extended engine off period in a hybrid electric vehicle are either by keeping the gasoline engine running or by running the electric motor, and both sacrifice fuel efficiency for passenger comfort. It is desirable to maintain both fuel efficiency and passenger comfort. Accordingly, a need exists for an extended engine off passenger climate control system and method.